18.4 Implementation-dependent characteristics

Although the Ada language defines the semantics of each construct as
precisely as practical, in some situations (for example for reasons of
efficiency, or where the effect is heavily dependent on the host or target
platform) the implementation is allowed some freedom. In porting Ada 83
code to GNAT, you need to be aware of whether / how the existing code
exercised such implementation dependencies. Such characteristics fall into
several categories, and GNAT offers specific support in assisting the
transition from certain Ada 83 compilers.